do-timestamp option doesn't work properly

[VLongobardi]

Example 1:

gst-launch-1.0 zmqsrc endpoint=tcp://127.0.0.1:50001 do-timestamp=true ! video/x-raw, format=I420, width=640, height=480, framerate=30/1 ! fpsdisplaysink video-sink=fakesink

Setting pipeline to PAUSED ...
Pipeline is PREROLLING ...
0:00:00.131577308  3983 0xaaaaf07ad300 FIXME                default gstutils.c:4025:gst_pad_create_stream_id_internal:<zmqsrc0:src> Creating random stream-id, consider implementing a deterministic way of creating a stream-id
0:00:00.157475986  3983 0xaaaaf07ad300 WARN         basetextoverlay gstbasetextoverlay.c:3049:gst_base_text_overlay_video_chain:<fps-display-text-overlay> buffer without timestamp, discarding
0:00:00.175540121  3983 0xaaaaf07ad300 WARN         basetextoverlay gstbasetextoverlay.c:3049:gst_base_text_overlay_video_chain:<fps-display-text-overlay> buffer without timestamp, discarding
0:00:00.186531106  3983 0xaaaaf07ad300 WARN         basetextoverlay gstbasetextoverlay.c:3049:gst_base_text_overlay_video_chain:<fps-display-text-overlay> buffer without timestamp, discarding

Example 2:

gst-launch-1.0 zmqsrc endpoint=tcp://127.0.0.1:50001 do-timestamp=true ! video/x-raw, format=I420, width=640, height=480, framerate=30/1 ! videorate ! video/x-raw, framerate=5/1 ! fakesink
Setting pipeline to PAUSED ...
Pipeline is PREROLLING ...
0:00:00.104167037  4399 0xaaaad89ccb60 FIXME                default gstutils.c:4025:gst_pad_create_stream_id_internal:<zmqsrc0:src> Creating random stream-id, consider implementing a deterministic way of creating a stream-id
0:00:00.132224210  4399 0xaaaad89ccb60 WARN               videorate gstvideorate.c:1789:gst_video_rate_transform_ip:<videorate0> Got buffer with GST_CLOCK_TIME_NONE timestamp, discarding it
0:00:00.147644262  4399 0xaaaad89ccb60 WARN               videorate gstvideorate.c:1789:gst_video_rate_transform_ip:<videorate0> Got buffer with GST_CLOCK_TIME_NONE timestamp, discarding it
0:00:00.165624864  4399 0xaaaad89ccb60 WARN               videorate gstvideorate.c:1789:gst_video_rate_transform_ip:<videorate0> Got buffer with GST_CLOCK_TIME_NONE timestamp, discarding it